1. Field of the Invention
The invention relates to an optical recording medium having a recording layer which makes an optical change for data recording when irradiated with laser light.
2. Description of Related Art
Optical recording media such as a compact disc (CD) and a digital versatile disc (DVD) are widely used as information recording media. In recent years, attention is being given to optical recording media which use blue or blue violet laser light as irradiation light so that a large amount of information can be recorded at still higher densities. For the sake of specification standardization, blue violet laser light having a wavelength of approximately 405 nm has been proposed for use, and compatible optical recording media are now becoming prevalent. When the blue or blue violet laser light is used as the irradiation light, tracks on an optical recording medium are formed at track pitches within the range of 0.1 to 0.5 μm. Incidentally, a plurality of recording layers may be formed with a transparent spacer layer(s) therebetween. This multilayer recording type allows a further increase in the recording capacity.
Optical recording media are broadly classified into a ROM (Read Only Memory) type in which data cannot be added or rewritten, an R (Recordable) type in which data can be added only once, and an RW (Rewritable) type in which data can be rewritten. The recording layer of R-type optical recording medium needs to make changes in optical characteristics when irradiated with laser light. Besides, it is essential that the recording layers be unsusceptible to deterioration even after a long period of storage, having excellent durability. Conventionally, organic dye has thus been used widely as a material of the recording layers of the R-type optical recording media. This conventional organic dye is a substance less likely to absorb ultraviolet rays and short-wave visible rays, such as blue and blue violet, which are prone to promote chemical reactions. It is this feature of the conventional organic dye that has contributed suppressed deterioration.
Since the conventional organic dye is less likely to absorb short-wave visible rays of blue and blue violet, however, it has been impossible to obtain satisfactory change in optical characteristics for data recording when the blue or blue violet laser light is used as the irradiation light. Moreover, it has been difficult to develop an organic dye which provides satisfactory change in optical characteristics even for situations where the blue or blue violet laser light is used as the irradiation light, and is unsusceptible to deterioration for a long period of storage.
In view of the foregoing, R-type optical recording media that have recording layers made of inorganic material containing Bi and O have been known(refer to, for example, Japanese Patent Laid-Open Publications Nos. 2003-48375 and Hei 10-334507).
Nevertheless, even the inorganic material containing Bi and O has sometimes failed to achieve desired change in optical characteristics when the blue or blue violet laser light is used as the irradiation light.
Besides, such an inorganic material containing Bi and O can vary in reflectance and in light transmittance as well, when irradiated with laser light. Consequently, if the inorganic material containing Bi and O is used to make the recording layers of an optical recording medium of multilayer recording type, the laser light to reach the lower(on the substrate side) recording layer varies in intensity between where the upper (on the cover-layer side) recording layer has been irradiated with the laser light to form recording marks and where not. There has thus been the problem that the accuracy of recording of data on the lower recording layer and the accuracy of reproduction of data from the lower recording layer are low.